Wafer-like processing after sawing DMDs

Abstract: A processing fixture and method of fabricating micromechanical devices, such as digital micromirror devices, that allows fragile structures on wafer 22 to be protected from debris during the saw operation and subsequent cleaning operations. The wafer 22 is attached to a vacuum fixture 26 after partially sawing the wafer 22 to create saw kerfs. The backside of the wafer 22 is then ground down to the saw kerfs 24 to separate the devices 32 . Each device 32 is held on the fixture by a vacuum in the headspace above the device 32 . In an alternate embodiment the devices are separated by sawing completely through the wafer while in the fixture.

Description: FIELD OF THE INVENTION This invention relates to the field of integrated circuit processing, more particularly to micromechanical device fabrication, including deformable micromirror devices. BACKGROUND OF THE INVENTION In order for integrated circuit (IC) processing to be cost effective, the individual circuits, or chips, should be mass produced by using a semiconductor wafer to make many chips on a single substrate simultaneously. Typical processing for microelectronic mechanical systems (MEMS) involves extensive chip handling during processes such as the sacrificial layer removal and device testing. The ability to perform these processes in wafer form as opposed to chip form is very attractive. Carrying out the processing at the wafer level reduces the handling necessary because processing equipment must only move and align one wafer instead of many chips. Alignment, or registration, is very critical for die testing. After all wafer level processing has been done, the chips are separated and packaged. When the devices are separated from a wafer, wafer particles and dust, also known as dicing debris, is created. This dicing debris is then typically washed from the surface of the IC prior to bonding the chip to the package. A semiconductor wafer dicing fixture is known from US-A-3,976,288. This document discloses apparatus for retaining a semiconductor wafer in a predetermined position or orientation to permit dicing of the wafer into discrete semiconductor chips of a predetermined size without dislocation of the chips during the dicing operation, the chips being arranged in columns and rows with a kerf area intermediate adjacent chips. The apparatus comprises a base member having a semiconductor wafer positioning area with a connection to the base member for applying the vacuum to a plurality of spaced apart apertures in the wafer receiving area, the apertures corresponding to the location of the chips. A cover plate, adapted for superimposition of the wafer receiving area, includes a plurality of interdigitated ribs and slots, the slots extending through the cover and dimensioned for alignment with the kerf area of the wafer, the ribs having strips of compressible material underlying the ribs for superimposed pressing relation on the wafer so that when a wafer is being cut through the slots along the kerf area, the vacuum being drawn coupled with the pressing engagement of the strips on the wafer inhibit disclocation of the chips during the dicing operation. Micromechanical devices often have structures that are too fragile to survive exposure to some of the standard IC fabrication steps. An example is the digital micromirror device (DMD). DMDs are explained in U.S. Patent No. 5,061,049 "Spatial Light Modulator and Method" which is assigned to Texas Instruments Incorporated. As described in the aforementioned patent, DMDs have a very small mirror suspended over an air gap above electrodes formed on the surface of a silicon substrate. Once this mirror is formed and the sacrificial material etched from the air gap, the DMD is very fragile. The devices cannot be exposed to liquids, for example during wafer cleanup steps, without destroying the mirror. Therefore, the devices must be cut and the dicing debris washed away before etching the sacrificial layer away from the mirror. This requires that the cleaning and etching steps, and any following steps, including testing be performed on the individual chips instead of a wafer. SUMMARY OF THE INVENTION In accordance with the present invention, a protective fixture as defined in claim 1 has been constructed to allow efficient wafer processing of micromechanical devices. The method of the invention comprises the features defined in claim 7 or claim 12. The fixture can be held against a wafer containing micromechanical devices by drawing a vacuum on the headspaces enclosed between the fixture and the wafer. The fixture is constructed with surfaces that allow the fixture and wafer to form a seal around each device. The devices on the wafer are then separated and any debris created during the separation process can be washed off of the devices and the fixture. It is an advantage of this invention to provide a fixture and method that allow a wafer containing micromechanical devices to be completely fabricated using industry standard techniques. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A and 1B are top and side views respectively of a wafer showing partially sawn streets separating devices substantially enlarged for illustration purposes. FIG. 2 is a schematic top view of a vacuum fixture according to a first embodiment of the present invention. FIG. 2A is a sectional view of the fixture of Figure 2 taken along line 2A-2A. FIG. 3 is a sectional view of a partially sawn wafer held against the vacuum fixture of Figure 2 prior to grinding the back side of the wafer. FIG. 4 is a sectional view of a partially sawn wafer held against the vacuum fixture of Figure 2 after grinding the back side of the wafer. FIG. 5 is a schematic top view of a vacuum fixture according to a second embodiment of the present invention. FIG. 5A is a sectional view of the fixture of Figure 5 taken alone line 5A-5A. FIG. 6 is a sectional view of a wafer held against the vacuum fixture of the embodiment of Figure 5 prior to the saw operation. FIG. 7 is a sectional view of a wafer held against the vacuum fixture of the embodiment of Figure 5 after the saw operation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1A and 1B show a silicon wafer 22 on which micromechanical devices, such as digital micromirrors are fabricated. However, it should be understood that this invention is applicable to other substrates and devices. Saw kerfs 24 are made on wafer 22 along scribe marks or streets between the individual devices prior to undercutting the mirrors on the devices. Although the devices shown in figures 1A and 1B are substantially enlarged for illustration purposes such that each figure only shows four rows of devices, each wafer would typically have a substantially larger number of rows. When other micromechanical devices, such as accelerometers or motors, are being fabricated, the partial saw kerfs 24 should be made prior to any fabrication step that makes the devices too fragile to survive cleaning the dicing debris from the device. After the dicing debris has been washed off, the mirrors are undercut. After the undercutting is completed, the mirrors are fragile and cannot be exposed to many manufacturing processes. Fabrication of the devices, including removal of the sacrificial layer and testing may be completed in wafer form. Figure 2 shows a vacuum fixture 26 according to one embodiment of the present invention. The fixture 26 may be stainless steel, ceramic, quartz, or any other material capable of being machined or formed to the necessary tolerances. The fixture 26 is machined to have a plurality of headspaces such as headspace 28 above the devices on the wafer. for example, if micromirror devices are being manufactured the headspace 28 above the mirrors will help prevent contact with the mirrors. The headspaces 28 above each device are connected to a plurality of vacuum ports 30 to allow the headspaces 28 to be evacuated. The fixture 26 includes separating members 29A, 29B, 29C, 29D and 29E suitably machined and located to cover the saw kerfs cut into the wafer and to seal around each device. When all process steps necessary to complete the devices on the wafer have been completed, the partially sawn wafer 22 is fitted to a vacuum fixture 26 as shown in Figure 3. Headspaces 28 are then evacuated through vacuum ports 30 and the backside of the wafer 22 is ground down to the partial saw kerfs in order to separate the devices. The operation of grinding the backside of the wafer also provides better thermal management on the completed device. An alternative technique to grinding the backside of the wafer would be to saw the wafers from the backside through to the saw kerfs 24 on the front side. Another alternative would be to use a tool designed to break the wafers. For certain of these alternative techniques, the fixture 26 may need to be flexible to allow some of the wafer breaking equipment to operate. Figure 4 shows the completed devices after the devices on the wafer have been separated. Each device is held by the vacuum against the fixture 26 such that the combination of the devices and fixture 26 may be washed to remove any debris from the separation operation. After washing, the vacuum holding the devices to the fixture 26 is removed to allow the device 32 to be removed from the fixture 26 . The completed devices are then ready for packaging and any processing further necessary. An alternate embodiment of the process of this invention does not require the scribe marks or streets to be partially sawn before completion of the devices. According to the alternate embodiment, shown in Figure 5, the fixture 36 has a plurality of machined passages 38 designed to align with the scribe streets of the wafer. Then, as shown in Figure 6, after the wafer 34 is completely fabricated and all wafer level processing steps, including testing, are completed, wafer 34 is mounted to fixture 36 by drawing a vacuum on the headspace through vacuum ports 40 . The devices on wafer 34 are separated by a backside sawing operation. As shown in Figure 7, each saw kerf 42 is aligned with a passage 38. As in the prior embodiment the wafer and fixture can now be washed to remove any dicing debris created by the sawing operation. The vacuum is then removed allowing the plurality of completed devices 44 to be removed from the fixture 36 . Thus, although there has been described to this point a particular embodiment for a method of micromechanical device fabrication in which fragile micromechanical structures are protected from debris during dicing, it is not intended that such specific references be considered as limitations upon the scope of this invention except in-so-far as set forth in the following claims. Furthermore, having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art, it is intended to cover all such modifications as fall within the scope of the appended claims.

Claims: A fixture for protecting a wafer (22; 34) comprised of at least two micromechanical devices, each micromechanical device comprised of at least one fragile portion, the fixture (26) comprising: a surface for contacting the wafer (22; 34), the surface comprised of a plurality of recessed regions for preventing contact between the surface and the wafer (22; 34) such that when the surface of the fixture (26; 36) is in contact with the wafer (22; 34), each recessed region of the surface forms a headspace (28) above the fragile portion of one of the micromechnical devices; at least one vacuum port (30; 40) connected to the headspaces (28) to allow evacuation of the headspaces (28); and the surface of the fixture (26; 36) forming a seal around each micromechanical device, thereby allowing each headspace (28) to be evacuated through the vacuum port (30; 40) connected to the headspace (28). The fixture of claim 1, the fixture (26; 36) comprised of a material selected from the group consisting of stainless steel, ceramic and quartz. The fixture of claim 1 or claim 2, wherein the fixture (26; 36) is rigid. The fixture of claim 1, claim 2 or claim 3 and further comprising passages (38) in the surface of the fixture (36), the passages (38) allowing a saw blade to separate the micromechanical devices from the wafer (34) without contacting the fixture (36). The fixture of claim 1 wherein the vacuum port is comprised of a separate vacuum port connected to each headspace (28). The fixture of claim 1 wherein the vacuum port is comprised of one vacuum port connected to all headspaces (28). A method of processing a wafer (22; 34) containing one or more micromechanical devices, said method comprising: placing a protective fixture (26; 36) over and in contact with a first side of said wafer (22; 34), said protective fixture (26; 36) defining a headspace (28) above each device; evacuating said headspace (28) to create a vacuum; grinding the side of said wafer (22; 34) opposite said first side to separate said devices; and removing said vacuum to release said devices from said fixture (26; 36). The method of claim 7 further comprising the step of partially sawing between said devices on said first side prior to placing said wafer (22) in said fixture. The method of claim 7 or claim 8 wherein said protective fixture (26; 36) is fabricated of a material selected from the group consisting of stainless steel, ceramic, and quartz. The method of claim 7 or claim 8 wherein said protective fixture (26; 36) is a flexible protective fixture. The method of any of claims 7 7 to 10 further comprising the step of cleaning debris created by said grinding step from said devices and said protective fixture (26; 36). A method of processing a wafer (22; 34) containing one or more micromechanical devices, said method comprising: placing a flexible protective fixture (26; 36) over and in contact with said wafer (22; 34), said protective fixture (26; 36) defining a headspace (28) above each device; evacuating said headspace (28) to create a vacuum; breaking said wafer (22; 34) to separate said devices; and removing said vacuum to release said devices from said fixture (26; 36).

Inventors: GALE RICHARD O, MIGNARDI MICHAEL A, GALE, RICHARD O., MIGNARDI, MICHAEL A.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1994
Main classification: G02B26
IPC codes: B28D5, G02B26, B25B11, B81C1, H01L21